Compositional Pointer and Escape Analysis for Java Programs

1
Compositional Pointer and Escape Analysis for
Java Programs

• Martin Rinard
• Laboratory for
• Computer Science
• MIT

John Whaley IBM Tokyo Research Laboratory
2

• Analysis
• Points-to information
• Escape information
• Optimizations
• Stack allocation
• Synchronization elimination

3
Outline

• Example
• Analysis Algorithm
• Optimizations
• Experimental Results
• Conclusion

4
Employee Database Example

• Read in database of employee records
• Extract statistics like max salary

5
Employee Database Example

• Read in database of employee records
• Extract statistics like max salary

Name
Salary
6
Computing Max Salary

• Traverse Records to Find Max Salary

Vector
John Doe
Name
Ben Bit
Jane Roe
45,000
Salary
30,000
55,000
max 0
7
Computing Max Salary

• Traverse Records to Find Max Salary

Vector
John Doe
Name
Ben Bit
Jane Roe
45,000
Salary
30,000
55,000
max 45,000
who John Doe
8
Computing Max Salary

• Traverse Records to Find Max Salary

Vector
John Doe
Name
Ben Bit
Jane Roe
45,000
Salary
30,000
55,000
max 45,000
who John Doe
9
Computing Max Salary

• Traverse Records to Find Max Salary

Vector
John Doe
Name
Ben Bit
Jane Roe
45,000
Salary
30,000
55,000
max 55,000
who Jane Roe
10
Computing Max Salary

• Traverse Records to Find Max Salary

Vector
John Doe
Name
Ben Bit
Jane Roe
45,000
Salary
30,000
55,000
max salary 55,000
highest paid Jane Roe
11
Coding Max Computation

• class EmployeeDatabase
• Vector database new Vector()
• Employee highestPaid
• void computeMax()
• int max 0
• Enumeration enum database.elements()
• while (enum.hasMoreElements())
• Employee e enum.nextElement()
• if (max lt e.salary())
• max e.salary() highestPaid e

12
Coding Max Computation
class EmployeeDatabase Vector database new
Vector() Employee highestPaid void
computeMax() int max 0 Enumeration enum
database.elements() while (enum.hasMoreElemen
ts()) Employee e enum.nextElement() if
(max lt e.salary()) max e.salary()
highestPaid e
13
Issues In Implementation

• Enumeration object allocated on heap
• Increases heap memory usage
• Increases garbage collection frequency
• Heap allocation is unnecessary
• Enumeration object allocated inside max
• Not accessible outside max
• Should be able to use stack allocation

14
Key Concept

• Enumeration object is captured in max
• Allocated inside computation of method
• Inaccessible to callers of method
• Within method, object does not escape to another
  thread
• So object is dead when method returns
• Can allocate captured objects on stack

15
How is Database Used?

• void printStatistics()
• BufferedReader r new BufferedReader(
• new InputStreamReader(System.in))
• EmployeeDatabase e new EmployeeDatabase(r)
• e.computeMax()
• System.out.println(max salary
  e.highestPaid)

16
Interesting Fact

• void printStatistics()
• BufferedReader r new BufferedReader(
• new InputStreamReader(System.in))
• EmployeeDatabase e new EmployeeDatabase(r)
• e.computeMax()
• System.out.println(max salary
  e.highestPaid)
• Only one thread accesses employee database
• But accesses are synchronized!
• Employee e enum.nextElement()

17
Synchronization in nextElement

• class VectorEnumerator implements Enumeration
• Vector vector
• int count
• Object nextElement()
• synchronized (vector)
• if (count lt vector.elementCount)
• return vector.elementDatacount
• throw new NoSuchElementException()

18
Synchronized Libraries

• nextElement is in the Java class library
• Java is a multithreaded language
• By default, libraries are synchronized
• Result lots of unnecessary synchronization for
  objects that are accessed by only one thread

19
Eliminating Unnecessary Synchronization

• Captured objects are accessible to only one
  thread
• Can eliminate synchronization on captured objects

20
Basic Idea

• Use pointer and escape analysis to recognize
  captured objects
• Transform program to
• Allocate captured objects on stack
• Eliminate synchronization on captured objects

21
Analysis Overview

• Interprocedural analysis
• Compositional analysis
• Driving Principle
• Explicitly represent potential interactions of
  method with its environment (callers and other
  threads)

22
Points-to Escape Graph in Example
void computeMax() int max 0 Enumeration
enum database.elements() while
(enum.hasMoreElements()) Employee e
enum.nextElement() if (max lt e.salary()) max
e.salary() highestPaid e
dotted outside

vector
elementData
solid inside
enum
database
highestPaid
this
e
23
Definitions node types

• NI inside nodes
• represent objects created within the computation
  of the method
• one inside node for each object creation site
  represents all objects created at site
• NO outside nodes
• represent objects created outside of the
  computation of the method

24
Definitions outside node types

• NP parameter nodes
• represent objects passed as incoming parameters
• NL load nodes
• one load node for each load statement in method
• represents objects loaded from an escaped node
• NCL class nodes
• node from which static variables are accessed

25
Points-to Escape Graph in Example
void computeMax() int max 0 Enumeration
enum database.elements() while
(enum.hasMoreElements()) Employee e
enum.nextElement() if (max lt e.salary()) max
e.salary() highestPaid e
dotted outside

vector
elementData
solid inside
enum
database
highestPaid
this
e
26
Points-to Escape Graph in Example
void computeMax() int max 0 Enumeration
enum database.elements() while
(enum.hasMoreElements()) Employee e
enum.nextElement() if (max lt e.salary()) max
e.salary() highestPaid e
dotted outside

solid inside
vector
elementData
enum
database
highestPaid
red escaped
white captured
this
e
27
Escaped nodes

• Escaped nodes
• parameter nodes
• class nodes
• thread nodes
• nodes in return set
• nodes reachable from other escaped nodes
• captured is the opposite of escaped

28
Dataflow Analysis

• Computes a points-to escape graph for each
  program point
• Points-to escape graph is a triple ltI,O,egt
• I - set of inside edges
• O - set of outside edges
• e - escape function

29
Dataflow Analysis

• Initial state
• I formals point to parameter nodes,
• classes point to class nodes
• O Ø
• Transfer functions
• I (I KillI ) U GenI
• O O U GenO
• Confluence operator is U

30
Intraprocedural Analysis

• Must define transfer functions for
• copy statement l v
• load statement l1 l2.f
• store statement l1.f l2
• return statement return l
• object creation site l new cl
• method invocation l l0.op(l1lk)

31

• copy statement l v
• KillI edges(I, l)
• GenI l succ(I, v)
• I (I KillI ) ? GenI

Existing edges
l
v
32

• copy statement l v
• KillI edges(I, l)
• GenI l succ(I, v)
• I (I KillI ) ? GenI

Generated edges
l
v
33

• load statement l1 l2.f
• SE n2 ? succ(I, l2) . escaped(n2)
• SI ?succ(I, n2,.f) . n2 ? succ(I, l2)
• case 1 l2 does not point to an escaped node (SE
  Ø)
• KillI edges(I, l1)
• GenI l1 SI

Existing edges
l1
f
l2
34

• load statement l1 l2.f
• SE n2 ? succ(I, l2) . escaped(n2)
• SI ?succ(I, n2,.f) . n2 ? succ(I, l2)
• case 1 l2 does not point to an escaped node (SE
  Ø)
• KillI edges(I, l1)
• GenI l1 SI

Generated edges
l1
f
l2
35

• load statement l1 l2.f
• case 2 l2 does point to an escaped node (SE ? Ø)
• KillI edges(I, l1)
• GenI l1 (SI ? n)
• GenO (SE f) n

Existing edges
l1
l2
36

• load statement l1 l2.f
• case 2 l2 does point to an escaped node (SE ? Ø)
• KillI edges(I, l1)
• GenI l1 (SI ? n)
• GenO (SE f) n

Generated edges
l1
f
l2
37

• store statement l1.f l2
• GenI (succ(I, l1) f) succ(I, l2)
• I I ? GenI

Existing edges
l1
l2
38

• store statement l1.f l2
• GenI (succ(I, l1) f) succ(I, l2)
• I I ? GenI

Generated edges
l1
f
l2
39

• object creation site l new cl
• KillI edges(I, l)
• GenI ltl, ngt

Existing edges
l
40

• object creation site l new cl
• KillI edges(I, l)
• GenI ltl, ngt

Generated edges
l
41
Method call

• Transfer function for method call
• Take points-to escape graph before the call site
• Retrieve the points-to escape graph from analysis
  of callee
• Map callee graph into caller graph
• Result is the points-to escape graph after the
  call site

42
Interprocedural Mapping

• Set up an abstract mapping between caller and
  callee
• outside nodes in the callee may refer to any
  number of inside nodes in the caller
• add all reachable inside edges from callees
  graph into callers graph
• outside edges from a node in the callee need to
  be added to the mapped caller node if it escapes

43
Interprocedural Mapping Example

• void printStatistics()
• BufferedReader r new BufferedReader(
• new InputStreamReader(System.in))
• EmployeeDatabase e new EmployeeDatabase(r)
• e.computeMax()
• System.out.println(max salary
  e.highestPaid)

44
Interprocedural Mapping Example
void printStatistics() BufferedReader r new
BufferedReader( new InputStreamReader(System.in)
) EmployeeDatabase e new EmployeeDatabase(r)
e.computeMax() System.out.println(max salary
e.highestPaid)
graph before call site

elementData
database
e
45
Interprocedural Mapping Example
void printStatistics() BufferedReader r new
BufferedReader( new InputStreamReader(System.in)
) EmployeeDatabase e new EmployeeDatabase(r)
e.computeMax() System.out.println(max salary
e.highestPaid)
graph before call site

elementData
database
e
callee graph

elementData
database
this
highestPaid
Enum object is not present because it was
captured in the callee.
46
Step 1 Map formals to actuals
void printStatistics() BufferedReader r new
BufferedReader( new InputStreamReader(System.in)
) EmployeeDatabase e new EmployeeDatabase(r)
e.computeMax() System.out.println(max salary
e.highestPaid)
graph before call site

elementData
database
e
callee graph

elementData
database
this
highestPaid
47
Step 1 Map formals to actuals
void printStatistics() BufferedReader r new
BufferedReader( new InputStreamReader(System.in)
) EmployeeDatabase e new EmployeeDatabase(r)
e.computeMax() System.out.println(max salary
e.highestPaid)
graph before call site

elementData
database
e
highestPaid
database
callee graph

elementData
48
Step 2 Match callee outside edges against caller
inside edges
void printStatistics() BufferedReader r new
BufferedReader( new InputStreamReader(System.in)
) EmployeeDatabase e new EmployeeDatabase(r)
e.computeMax() System.out.println(max salary
e.highestPaid)
graph before call site

elementData
database
e
highestPaid
database
callee graph

elementData
49
Step 2 Match callee outside edges against caller
inside edges
void printStatistics() BufferedReader r new
BufferedReader( new InputStreamReader(System.in)
) EmployeeDatabase e new EmployeeDatabase(r)
e.computeMax() System.out.println(max salary
e.highestPaid)
graph before call site

elementData
database
e
elementData
callee graph

50
Step 2 Match callee outside edges against caller
inside edges
void printStatistics() BufferedReader r new
BufferedReader( new InputStreamReader(System.in)
) EmployeeDatabase e new EmployeeDatabase(r)
e.computeMax() System.out.println(max salary
e.highestPaid)
graph before call site

elementData
database
e
elementData
callee graph

51
Step 2 Match callee outside edges against caller
inside edges
void printStatistics() BufferedReader r new
BufferedReader( new InputStreamReader(System.in)
) EmployeeDatabase e new EmployeeDatabase(r)
e.computeMax() System.out.println(max salary
e.highestPaid)
graph before call site

elementData
database
e

highestPaid
callee graph
52
Step 2 Match callee outside edges against caller
inside edges
void printStatistics() BufferedReader r new
BufferedReader( new InputStreamReader(System.in)
) EmployeeDatabase e new EmployeeDatabase(r)
e.computeMax() System.out.println(max salary
e.highestPaid)
graph before call site

elementData
database
e

highestPaid
callee graph
53
Step 2 Match callee outside edges against caller
inside edges
void printStatistics() BufferedReader r new
BufferedReader( new InputStreamReader(System.in)
) EmployeeDatabase e new EmployeeDatabase(r)
e.computeMax() System.out.println(max salary
e.highestPaid)
graph before call site

elementData
database
e
highestPaid
callee graph
54
Step 3 Add nodes and edges and update escape
function
void printStatistics() BufferedReader r new
BufferedReader( new InputStreamReader(System.in)
) EmployeeDatabase e new EmployeeDatabase(r)
e.computeMax() System.out.println(max salary
e.highestPaid)
graph before call site

elementData
database
e
highestPaid
55
Step 3 Add nodes and edges and update escape
function
void printStatistics() BufferedReader r new
BufferedReader( new InputStreamReader(System.in)
) EmployeeDatabase e new EmployeeDatabase(r)
e.computeMax() System.out.println(max salary
e.highestPaid)

final graph
elementData
database
e
highestPaid
Escaped nodes and edges in callee may be
recaptured in caller!
56
Life is not so Simple

• Dependences between phases
• Mapping best framed as constraint satisfaction
  problem
• Solved using constraint satisfaction

57
Algorithm Features

• Partial program analysis
• can analyze libraries independent of callers
• can analyze a method without analyzing the
  methods it invokes
• Appropriate for a dynamic compiler
• Support for partial program analysis
• Analyze libraries offline and use results

58
Stack Allocation of Captured Objects

• Objects that are captured can be allocated in
  stack frame of capturing method

Stack before
max 0
obj header
enum
vect null
e null
index 0
fp
59
Stack Allocation of Captured Objects

• Objects that are captured can be allocated on the
  stack

Stack before
Stack after
max 0
max 0
obj header
obj header
enum
vect null
vect null
e null
index 0
index 0
fp
e null
fp
60
Stack Allocation of Captured Objects

• Stack allocation can extend lifetime of objects
• Allocations of arbitrary size are not transformed
• Allocation sites inside loops
• Arbitrary-sized arrays

61
Object captured within a caller

• Call path to the allocating method is specialized
• Add an extra parameter location in the callers
  stack frame where to put the object
• Object allocation is changed to allocate the
  object at the given location, rather than on the
  heap

62
Specialization policy

• Distance in the call graph between capturing
  method and allocating method may be large
• Implemented solution full specialization
• Specialized versions of all methods on call chain
  from capturing method to allocating method
• Does not seem to blow up in practice

63
Removing Synchronization on Captured Objects

• Identify lock-unlock pairs that can only operate
  on captured objects
• Augment analysis with sets of nodes for each
  lock-unlock pair
• If all nodes in a set are captured, then the
  lock-unlock pair can be removed

64
Experimental Results

• Implemented in the Jalapeño JVM
• Analysis performed on the entire VM
• Including the analysis code itself!

65
Synch Operations Eliminated
66
Stack Allocated Objects
67
Size of Stack Allocated Objects
68
Related Work

• Pointer Analysis for Sequential Programs
• Chatterjee, Ryder, Landi (POPL 99)
• Sathyanathan Lam (LCPC 96)
• Steensgaard (POPL 96)
• Wilson Lam (PLDI 95)
• Emami, Ghiya, Hendren (PLDI 94)
• Choi, Burke, Carini (POPL 93)

69
Related Work

• Pointer Analysis for Multithreaded Programs
• Rugina and Rinard (PLDI 99) (fork-join
  parallelism, not compositional)
• We have extended our points-to analysis for
  multithreaded programs (irregular, thread-based
  concurrency, compositional)
• Escape Analysis
• Blanchet (POPL 98)
• Deutsch (POPL 90, POPL 97)
• Park Goldberg (PLDI 92)

70
Related Work

• Synchronization Optimizations
• Diniz Rinard (LCPC 96, POPL 97)
• Plevyak, Zhang, Chien (POPL 95)
• Aldrich, Chambers, Sirer, Eggers (SAS99)
• Blanchet (OOPSLA 99)
• Bogda, Hoelzle (OOPSLA 99)
• Choi, Gupta, Serrano, Sreedhar, Midkiff (OOPSLA
  99)

71
Conclusion

• Combined points-to and escape analysis
• Interprocedural
• Compositional - Inside and outside nodes/edges
• Implemented in Jalapeño
• Encouraging experimental results